1. Field of the Invention
The present invention generally relates to a system for manufacturing printed circuit boards and a method of manufacturing the same, and more particularly, to a system and a method for the manufacture of the printed circuit boards suitable for effectively manufacturing different kinds of printed circuit boards in small numbers.
2. Related Art
Electronic equipment such as communication equipment generally comprises printed circuit board units (or electronic circuit packages) loaded into a shelf and consisting of printed circuit boards and electronic components mounted thereon. In the field of making industrial products such as communication equipment, particularly, the diversification of the customers' needs requires a flexible manufacture of the printed circuit boards, which inevitably leads to an increase in kinds of printed circuit boards and to a demand for the establishment of the way of effectively manufacturing different kinds of such products in small numbers.
A typical production control system includes a conveyance means such as a driverless, automatic conveying vehicle travelling between a plurality of processing or manufacturing cells or units (processing equipment) and storage units for storing products, a process controller for controlling the processing units, a conveyance controller for the control of the conveyance means, and a host computer which interchanges information with the above-mentioned controllers and issues instructions to the controllers in accordance with a work planning and a process sequence. The production control system called FMS (Flexible Manufacturing System) presents a production system capable of coping with a flexible manufacture and executing desired operations while passing a variety of products between the processing cells in accordance with the predetermined procedure.
The conventional methods of manufacturing printed circuit boards include an in-line system in which the processing units are placed in series by way of conveyors, and a parallel line system in which the processing units are arranged in parallel and the printed wiring boards are loaded into or unloaded from the processing units by means of the automatic conveying vehicles.
FIG. 1 schematically shows a conventional in-line system having a plurality of processing cells or units 1a and 1b linked in series by conveyors 2. A rack 3 containing a plurality of printed circuit boards on which electronic components are to be mounted, is loaded onto the conveyor 2 through a loader 4, and is conveyed by the conveyor 2 in the direction shown by arrow A. In the processing cells or units 1a and 1b, the printed circuit boards are removed from the rack 3 for the predetermined operations, and then are passed into the downstream processing units by conveyor 2. The printed circuit boards which have been thus completed through the variety of processing steps are discharged by an unloader 5.
FIG. 2 diagrammatically shows a conventional parallel line system having a plurality of processing cells or units 6a, 6b, . . . , 6n arranged in parallel along which a conveyance passage 7 extends. An automatic conveying vehicle 8 travels on the conveyance passage 7. A loader/unloader 9 designed to load and unload the rack 3 containing a plurality of printed circuit boards is provided on each of the processing cells or units 6a, 6b, . . . , 6n.
The rack 3 containing a plurality of printed circuit boards is mounted on the automatic carrying vehicle 8 through the loader 4, and brought into any one of the processing cells or units 6a, 6b, . . . 6n by way of their respective loader/unloaders 9 in accordance to a working procedure. In the processing cells or units 6a, 6b, . . . 6n, the printed circuit boards are dispensed from the rack 3 to undergo a predetermined operation and returned into the rack 3. The printed circuit boards are then transferred to the subsequent processing cells or units 6a, 6b, . . . , 6n on the basis of the working scheme. After the completion of all the processing operations, the finished printed circuit boards are ejected through the unloader 5.
Referring to FIG. 3, there is shown another conventional example of the parallel line system. Across a plurality of processing cells or units 12a to 12e arranged in parallel, is provided a passage 14 on which an automatic driverless vehicle 15 travels for carrying and loading workpieces, such as printed circuit boards, into the processing cells or units 12a to 12e. The processing cells 12a to 12e include their respective loaders/unloaders 13 for loading and unloading of the workpieces. Perpendicular to the passage 14, there extends a second passage 16 on which a stacker crane 17, having an automatic loading/unloading function, travels. Storage units 18 are disposed along each side of the second conveyance passage 16.
In the conventional parallel line system as shown in FIG. 3, the workpieces, such as printed circuit boards, stored within the storages 18, are dispensed onto the automatic vehicle 15 with the aid of the stacker crane 17, and transported to the desired processing cells 12a to 12e.
However, the in-line system shown in FIG. 1 and the parallel line system shown in FIG. 2 are both production lines having only processing cells or units, and hence involve the following problems.
(1) For the flexible manufacture, both manufacturing systems require that the processing or manufacturing units should be constructed to enable an arrangement or a replacement and replenishment of electronic components to be mounted on the printed circuit boards when changing the types of the printed circuit boards being made. At the time of such change in kinds of the products, e.g., printed circuit boards, the processing units are deenergized and hence the operating time of these units is adversely reduced. In the case of a chip component mounting machine, for example, the actual operating time thereof is in the order of 50 through 60% of the entire manufacturing time, with the remainder spent on the arrangement of the components or the like.
(2) Although the printed circuit boards are manufactured by the processing units in accordance with the manufacturing process, the processing time in each of the processing units at each processing step is not fixed due to the different requirements of operation. As a result, the processing units in less loaded processing steps remain idle and have a long waiting period for subsequent workpieces to arrive and be processed, which disadvantageously results in poor productivity of the production line.
On the other hand, the conventional production line shown in FIG. 3 has a problem that the storage units and the automatic vehicle must be separately and systematically controlled, which leads to a complicated control as well as higher production costs of the system. There also arises another problem that the distance of conveying the printed circuit boards can be long, and the productivity of the system is impaired in case the workpieces are to be temporarily withdrawn into the storage u its due to the fact that the workpieces cannot be conveyed between the processing units. Moreover, it takes an excessive time to transfer the workpieces when it is required that the workpieces be taken out from the storage units, because they are transferred by the automatic vehicle after the movement of the stacker crane.
In addition, the conventional automatic conveying vehicles travel at a relatively slow running speed of the order of 30 m/min. due to the travel on the floor in the factory, which results in further deficiency of the conveying process.